It's been a month now since Navigon offered what they call Panorama View 3D for MobileNavigator for the iPhone. Essentially, Navigon has taken NASA terrain and elevation data and added it to the map display. If you are driving in the mountains, you see the mountains on screen.
Of course, when you're driving, you shouldn't be staring at the pretty maps, but keeping your eyes on the road instead. At least the passengers can get entertained by Panorama View 3D.
Panorama View 3D is a U.S. $9.99 update to the Navigon app you already have. It currently works on the USA and Canada versions of Navigon MobileNavigator. It also reportedly works on the less expensive regional versions, but I was not able to test this. You purchase the Panorama View 3D module from the 'Extras' menu in the app.
I did notice that the map display seemed noticeably slower to respond, likely because of the greatly increased number of data points that have to be calculated and redrawn. I think it's a good trade-off though, for a spiffier map and an almost real-world view of your surroundings.
Ultimately, buying this add-on depends on where you live and where you travel. If you're mostly hanging around places like Omaha it doesn't have much use. If, however, your traveling takes you to hill and dale, the Navigon Panorama view is a hot ticket.